Kevin Matsuba, age 14, of Vancouver, B.C., Canada, for his question:

WHAT IS THE DOPPLER EFFECT?

The Doppler effect is the apparent change in frequency of sound, light or radio waves which is caused by motion. The pitch of a train whistle, for example, seems higher when the train approaches and lower after it passes and begins to move away. The actual pitch remains constant, but the Doppler effect gives the feeling of a change.

Astronomers study the speed of a star by measuring the apparent change in the frequency of its light waves due to motion.

A German physicist by the name of Christian Doppler first described the theory of the effect in 1842.